1. Field of the Invention
The present invention relates to a radar apparatus that transmits an electromagnetic wave and detects an object with equal angular spaces, and more particularly, to a radar apparatus that is installed in a vehicle and detects an object.
2. Description of the Related Art
Proposal is made of a radar apparatus that is installed at the front of a vehicle and measures a distance to an object and a relative velocity of the vehicle to the object. By measuring the distance and the relative velocity to the object, an alarm device can be activated to forewarn against a collision with the object or a brake is activated to avoid the collision with the object.
Among conventional radar apparatuses is an apparatus that rotates at a predetermined frequency a movable radar sensor that transmits a beam of an electromagnetic wave of a millimeter wave range and reciprocates the sensor within a certain angular range. This apparatus detects the object by transmitting the beam to the object existing at a predetermined angle and a predetermined distance in front of the vehicle and by receiving the reflected wave. Such apparatus is disclosed in Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 11-52042).
An average of the strength of the wave reflected by the object is calculated and output for each of equal angular spaces. The average of the strength of the reflected wave in each angular direction is compared with the averages of the strength at two adjoining angular directions and if the average of the strength in such angular direction is stronger than the averages of the strength in the two adjoining angular directions, then it is determined that such angular direction is the angular direction in which the object exists.
FIG. 1 is a diagram of detection of the object by the conventional radar apparatus. FIG. 2 is a diagram of details of angles in obtaining the average of the strength of the reflected wave at the equal angular spaces.
A radar apparatus 1 is installed at the front of a vehicle 2 and transmits the millimeter wave at the angle of +10° to −10° if a centerline of the vehicle is set at 0°. At this moment, the average of the strength of the reflected wave is obtained for the range of ±0.6° in the angular direction shown in FIG. 2.
For example, the average of the strength of the reflected wave at +6.6° in FIG. 2 is obtained by observing the reflected wave from +7.2° to +6.0°. Thereafter, the radar sensor rotates and the average of the strength of the reflected wave at +5.4° is obtained by observing the reflected wave from +6.0° to +4.8 °.
FIG. 3 is a diagram of an example of strength of the reflected wave illustrating the angular direction in which the object reflecting the transmitted wave exists. The strength of the wave reflected by the object is obtained at the angles shown in FIG. 2. Thus obtained strength of the reflected wave in each angular direction is compared with the strength in two adjacent angular directions and if the strength in any angular direction is stronger than the strength in two adjacent angular directions, then it is judged that the object exists in such angular direction. In the case of FIG. 3, since the strength of the reflected wave at −2.4° is stronger than the strength of the reflected wave at −3.6° and −1.2°, it is judged that the object exists in the direction of −2.4°. If the strength of the reflected wave in any angular direction does not exceed a threshold in FIG. 3, then it is judged that nothing has been observed in such angular direction.
However, in such the radar apparatus, at the end of a predetermined angular range for detecting the object, comparison can not be made with the strength of the reflected wave in two adjacent angular directions.
Description will now be made of the case where comparison can not be made of the strength of the reflected wave in certain angular direction with the strength of the reflected wave in two adjacent angular directions.
FIG. 4 exemplifies the case where the object exists at the end of the object detecting range. FIG. 5 exemplifies the strength of the reflected wave obtained by the observation in FIG. 4.
In FIG. 4, the radar apparatus 1 installed in the vehicle 2 detects the object by transmitting the millimeter wave to the object detecting range 3 and receiving the reflected wave. At this moment, a vehicle 4 running ahead moves in the direction of an arrow X and enters the lane on which the vehicle 2 is running. A part of the vehicle 4 comes into the object detecting range 3.
At this moment, the strength of the reflected wave as observed by the radar apparatus 1 is as shown in FIG. 5. The radar apparatus 1 detects nothing from the left end of the object detecting range to the angular direction of +4.8°, since the strength of the reflected wave does not exceed the threshold in such a range. Then, by the observation at +6.0°, the radar apparatus 1 detects for the first time the reflected wave exceeding the threshold. Thereafter, the strength of the reflected wave continues to rise at +7.2° and at +8.4° and reaches its peak at +9.6°.
The conventional radar apparatus compares the strength of the reflected wave in each angular direction with the strength of the reflected wave in two adjacent angular directions, and if the strength is stronger in any angular direction than in two adjacent angular directions, then it determines that such direction is the direction in which the object exists. For this reason, the radar apparatus can not determine the position of the object in the example of FIGS. 4 and 5. The case that fits in with the example of FIGS. 4 and 5 is, for example, an intrusion by other vehicle into the lane on which a vehicle is running.
Detailed description will be made of the strength of the reflected wave in the situation of FIG. 4.
FIGS. 6A to 6C are diagrams of the strength of the reflected wave at the time of intrusion of the vehicle from an adjacent lane to the right. When the vehicle intrudes from the adjacent lane to the right as shown in FIG. 4, the reflected wave is observed only at the end of the detecting range 3 as shown in FIG. 6A. A solid line RL shown on the vehicle 4 running ahead indicates the strength of the reflected wave and a broken line BL indicates the strength of the reflected wave that would be observed if the reflected wave were within the detecting range 3.
In FIG. 6A, the reflected wave is observed only at the end of the detecting range 3. In the situation of FIG. 6A, the peak can not be definitely determined, and the conventional radar apparatus does not recognize the intruding vehicle. FIG. 6B shows the state in which, after a lapse of certain time from FIG. 6A, the vehicle has further intruded. Even in this diagram, the peak of the strength of the reflected wave can not be definitely determined, and the conventional apparatus does not recognize the intruding vehicle. FIG. 6C shows the state in which, after a lapse of certain time from FIG. 6B, the vehicle has further intruded. In this diagram, the strength of the reflected wave at the end of the detecting range is weaker than the strength of the reflected wave nearest at the inner side thereof, and the peak of the strength of the reflected wave can be confirmed.
As seen above, when the angular direction can not be determined although the reflected wave is detected, the processing associated with the detection of the object can not be executed. In particular, determination can not be made of the angular direction of the vehicle suddenly intruding into the lane on which the vehicle in question is running, at the very moment of the intrusion. As a result, it takes time until the object is detected and an appropriate processing can not be made or uneasiness is caused to a driver of the car. Furthermore, if the detection takes time, the car collides with the object, in particular, the intruding vehicle.